The power system is generally configured to include a plurality of generators with rotors, transmission and distribution lines, loads, a voltage/current sensor for detecting a voltage and a power flow of a transmission and distribution network, a protection relay, a circuit breaker for turning ON/disconnecting a generator and a transmission line for the transmission and distribution network in operation by a switching operation, and a power system stabilizing system for controlling the circuit breaker by receiving as inputs thereof operation signals of the voltage/current sensor and the protection relay. As a result, when a ground fault or a line-to-line short circuit fault has occurred in the load or the transmission and distribution lines, the protection relay is activated due to an overcurrent detected by the current sensor to carry out a circuit breaker opening operation for removing fault point from the grid.
When the system voltage gets lower due to the grid fault, an active power which can be supplied from the generator to the load is reduced. When the active power which is to be transmitted is reduced, a breaking torque in the generators' rotor is also reduced. On one hand, since the adjustment of a mechanical input of the generator is accompanied by the delay, the mechanical input exceeds the breaking torque, and as a result, for a period of time required for the removal of the grid fault, rotating speed of the rotors in the generators increases.
On the other hand, when the grid fault is removed by the opening operation of the circuit breaker, since the voltage of the transmission and distribution line returns, the active power which is to be supplied to the transmission and distribution line returns. As a result, the breaking torque returns. Due to the returning of the breaking torque, the angular frequency of the rotor is reduced to settle into an angular frequency corresponding to a commercial frequency.
However, in the case where the voltage drop during grid fault is remarkable, or in the case where a period of time up to the disconnection of the fault point from the grid is long, the rotors are accelerated so much that the generator can have step-out in some cases.
For the stabilization of the rotor speed after the grid fault, the sufficient electrical load is required. For this reason, for the purpose of avoiding the major power outage, there is adopted a technique in which an electrical load of a generator group for which an interconnection state with the system is maintained by disconnecting some generators from the system is increased to keep the stability of whole system.
As described above, the generator stability (transient stability) after the fault depends on the terminal voltage of the generators, and the duration of grid fault. Therefore, the power system stabilizing system is previously provided with a circuit breaker opening table (hereinafter referred to as a control table) in which a portion and a form of the generation of the grid fault are supposed. Thus, after the generator output and the power flow state just before the grid fault are grasped, at the time of the generation of the grid fault, the power system stabilizing system receives as an input thereto operation information associated with the protection relay, and speedily opens the corresponding circuit breaker in accordance with the table described above, thereby preventing wide-area power outage.
Patent Document 1 discloses a control technique with which for the opening of the circuit breaker, the generator transient stability is evaluated by using both off-line information and on-line information, thereby disconnecting the generator. In addition, Non-Patent Document 1 discloses a technique with which state estimation calculation for estimating a state of the power system through calculation from limited sensor information is carried out, thereby controlling the generator disconnection by using the calculation result.